You can, but the type of enlargement you will capture is probably not the type of enlargement you are seeking. Synthol is a fatty acerbic that many bodybuilders inject directly into their muscles to construct them look bigger, and it works because the fatty substance is temporarily trapped between the muscle fibers so it puffs them up and makes them look huge.

However, the penis is made up of spongy tissue, not muscle. Injecting synthol into this tissue would create it to swell, blocking both the urethral opening and blood flow. So, even though the penis itself might look bigger, you resembling will had trouble maintain an erection, and passing urine and semen. And, you would manifestly sustain (possibly permanent) damage to the tissues surrounded by the penis because they are delicate and can be slickly damaged if the penis remains distended too long. Unless you do not love your penis, and don't mind not human being able to hold a normal sex duration afterwards, this is definitely not something I would recommend.